Developer composition for developing an electrostatic latent image



Patented May 12, 1953 UNITED STATES PATENT OFFICE DEVELOPER COMPOSITION FOR DEVELOP- ING AN ELECTROSTATIC LATENT IMAGE tlon of Delaware No Drawing. Application May 1, 1948, Serial No. 24,674

9 Claims.

This invention relates to a method and material for making powder images electrically, adapted especially for making and developing electrophotographic images, and has for its purpose to improve the art of electrophotography to an extent that enables producing satisfactory, commercially successful reproductions which are equal or superior to those made by conventional photocopy methods, affording clear, sharp, perfect reproductions free from blemishes and defects at a much lower cost than by usual photocopy processes, and to accomplish these results in a manner that does not require expert knowledge or operation but which can be performed readily with little experience.

It has been proposed to obtain an electrostatic image on an electrostatically charged insulating plate, as for instance in Patents Nos. 2,221,776, November 19, 1940, and 2,297,691, October 6, 1942, as by coating a conductive metal plate with a photoconductive insulating material such as sulphur, anthracene, or anthraquinone, and charging the surface of the coated metal plate by rubbing its surface, or by electrical induction, or by the action of either an electrostatic generator or a transformer-rectifier system, and after exposing such charged plate to light and the subject to be reproduced, the areas where light is received on the plate are discharged and the areas where light is not received remain charged corresponding somewhat to the light and darker areas of the subject. The electrostatic image thus produced is developed by bringing in contact with the late a suitable dusting powder known as an electroscopic powder which functions to develop the image by adhering to the charged areas of the plate in quantities roughly proportional to the amount of charge on each particular area.

Various objections and difliculties have arisen to prevent successful operation of this process prior to the present invention, among which is to be mentioned a condition of the electroscopic powder which when sufliciently fine for proper image development has a tendency to form into balls or clots which result in streaks and smears as they flow across the plate. Some powder particles adhere to an excessive extent to discharged areas of the plate, other powder particles are deposited to a greater extent in some portions of the charged areas than others, resulting in a lack of uniformity of the electroscopic powder layer over a uniformly charged area, and it is a pura way that streaks and smears from balls or clots of powder are eliminated, a uniform powder image is obtained that is always consistently in accord with the dark and light areas of the sublect, and a uniform amount of electroscopic powder adheres to a uniformly charged area of the plate while the discharged areas of the plate are left free of the powder, proportionately to the amount of charge remaining on the charged areas and the amount of light that has been projeoted on to the discharged areas of the plate, thus eliminating adherence of electroscopic powder to discharged areas and obviating excessive deposition on the charged areas.

In a more specific aspect, the invention contemplates the use of an electroscopic powder and a granular carrier which by mixing acquire electric charges of opposite polarities, the granular carrier functioning to insure removal of the electroscopic powder from the uncharged areas of the plate, coupled with an improved method of electrostatically charging the photoconductive insulating plate by means of corona discharge from a power source that produces a high voltage between points such as needles or wires, and the plate, the needles or wires being uniformly spaced from each other and uniformly spaced from the plate and distributed thereover.

This insures an equal and uniform charge over the entire surface of the plate and makes it possible to apply to the plate a charge of much higher potential than heretofore, and without affecting the image by rubbing, abrading, or otherwise inluring the surface of the plate, thus retaining the electroscopic powder uniformly in the charged areas with certainty while providing a granular carrier that acquires upon mixing with the electroscopic powder a charge of such potential as to insure removing the electroscopic powder from the discharged areas of the plate, and at the same time making it possible through proper selection of the electroscopic powder and granular carrier to control accurately the contrast of the reproduction by governing the relationship between the polarity and potential of the charges on the plate, the electroscop'ic powder, and the granular carrier, so as to bring about any desired result.

In carrying out the invention, a metal plate that is grounded and has applied to it a suitable photoconductive insulating layer is charged electrostatically by corona discharge from a suitable power source to produce a voltage between the needles or wires and plate of from 4500 to 20,000 volts by positioning the plate or photoconductive insulating layer in charging relation to a multiplicity of needles or Wires uniformly spaced from the plate and from each other, and passing therethrough a suitable current to effect a corona discharge from the needles or wires on to the plate, the plate and needles or wires being moved relatively past one another if preferred, and by thus uniformly discharging such a current, there is produced a high potentia1 electrostatic charge distributed uniformly over the surfaces of the plate or photoconductive insulating layer in such a way that the surface of the plate is not scratched or marred in any way, but may be used many times without its emciency being lessened. If wires are employed for charging, such wires may be arranged parallel to each other and parallel to and uniformly spaced from the surface of the plate to be charged.

The voltage of the charge resulting on the surface of the plate depends" on the spacing of the needles or wires from each other andfrom the plate, and any suitable highvoltage source of cur-- rent, either D. 0., A. (3., or rectified A. C., may be employed, just so there is sumcient potential between the needles and plate to give the desired charge on the plate. The voltage between the needles and plate is preferably slightly less than what would cause arcing and the amount of current small, possibly one milliampere or less, although the amount of current is not critical.

The conditions may vary somewhat dependent upon whether the charge on the electrophotographic plate is positive or negative. For example, with a negative charge, when the charging needles are separated from the plate 0.375" and the spacing between the needles is 0.320", the charging voltage should be about 5000 volts, while with a negative charge on the plate, a separation between the charging needles and plate of 0.250", and a spacing between the needles of 0.160", a charging voltage of about 4500 volts is satisfactory. With a negative charge on the plate, a separation between the charging needles andplate of 0.500", and spacing between the needles of 0.320", the charging voltage should be about 8000 volts. (All voltages specified above are peak readings for half-wave rectified A. C.)

With a positive charge on the electrophotographic plate, a separation between the charging needles and plate of 0.250, and spacing between the needles of 0.160", the charging voltage should be about 6000 volts, while with apositive charge on the plate, a separation between the charging needles and plate of 0.375, and a spacing between the needles of 0.160", the charging voltage should be about 8000 volts.

The above are examples of practical charging conditions although the invention is not limited to these particular spacings between the needles and plate or the charging voltages mentioned, provided the necessary conditions are maintained to. effect on the plate a charge having a polarity and potential in such relationship to the charges acquired by the electroscopic powder and granular material respectively upon mixing as to bring about the results contemplated bythe invention.

The charge on the plate or photoconductive insulating layer has a potential of from 100 volts to 700 volts or higher, and after the plate has been exposed to light and the electrostatic charge conducted away in the lighted areas proportionately to the amount of light striking such areas, the potential over the discharge areas drops to about 50 volts or less, which is somewhat less than the potential of the charge acquired by 4 the granular carrier forming part of the developing material, as will be described more fully presently. l

The potential of the charge has been determined by comparing the galvanometer deflection caused by the charged plate with the galvanometer deflection produced by a metal plate connected to a potential source and adjusting the latter potential until the galvanometer defiection for the metal plate is the same as for the charged plate. The potential necessary to be applied to the metal plate to produce a galvanometer deflection corresponding to that produced by the charged plate is considered to be the same as the potential of the charge on the charged plate.

After the photoconductive insulating layer has been thus provided with a high potential electrostatic charge, it is exposed to light to project the image of the subect to be reproduced on to the plate and thereby form the electrostatic the image of the subject to be reproduced on overthe plate in any suitable manner, as by tipping the plate or otherwise, a material that includes an electroscopic powder which acquires a charge having'a polarity opposite to that of the electrostatic charge on the plate, and which adheres uniformly to the areas of the plate that have not been affected by light and therefore remain charged, while the powder does not adhere to areas which have been electrostatically discharged by light striking them, thus forming a powder image on the plate corresponding to the electrostatic image and to the subject, and this powder image is subsequently transferred to the paper or transfer material on which the reproduction is to be made.

The developing material forming part of this invention is of a character that insures its adhering to or being released from the areas on the plate, in close accordance with and proportionately to the intensity of light that has hit such areas, thus producing a powder image conforming more closely to the subject to be reproduced than has heretofore been possible, and bringing about a clearer and sharper reproduction with more definite contrast, free from streaks, spots, blurred areas,-or other imperfections. 1

This is accomplished by first producing an electroscopic powder consisting of particles of pigmenting or coloring material encased in or surrounded by an insulating material which acquires by contact with the granular material, to be described presently, an electrostatic charge having a polarity opposite to that acquired by the granular'material and opposite to that of the photoconductive insulating layer of the plate.

The coloring material may be carbon or other suitable pigments, and the insulating material may be a rosin-modified phenol-formaldehyde resin such as known commercially as Amberol F-7l, and manufactured by Rohm and Haas Company, The Resinous Products Division, Washington Square, Philadelphia 5, Pa, or asphaltum or other suitable material.

The electroscopic powder is prepared by first micronizing the insulating or resin material such as Amberol F-7l, after which it is mixed with approximately 5% by weight of carbon black or other pigmenting material and the mixture ball the pigmenting particles with the Amberol F-7l.

The mass is then permitted to cool, after which it is broken into small chunks and again micronized.

The electroscopic powder is then in condition for mixing with the granular carrier such as polymerized methyl methacrylate having a melting point or approximately 257 F., known commercially as Lucite and manufactured by E. I. du Pont de Nemours 8: Company, Wilmington, Del., or other material either conducting or insulating, provided the particles of granular material when brought in close contact with the electroscopic powder particles acquire a charge having an opposite polarity to that of the electroscopic powder particles such that the electroscopic powder particles adhere to and surround the granular carrier particles. The granular carrier material is selected so that the particles acquire a. charge having the same polarity as that of the photoconductive insulating layer of the plate on which the electrostatic image is produced, and an electrical attraction for the electroscopic powder particles considerably less than that of the charged areas of the plate and somewhat greater than the discharged areas of the plate.

The granular carrier particles are larger than the electroscopic powder particles, and it has been found in practice that successful results are had with granular carrier particles of a size between 30 and 60 mesh and electroscopic powder particles of a size of from .10 to 20 microns, although the granular carrier particles may be somewhat larger or smaller as long as a proper size relationship to the electroscopic powder is maintained so that the granular carrier particles will flow easily over the plate by gravity when the plate is tipped without requiring tapping or air-pressure to move the granular carrier particles over the plate.

It is essential that the granular carrier particles be of sufficient size so that their gravitation or momentum force is greater than the force of attraction of the electroscopic powder in the charged areas where the powder is retained on the plate, so that the granular carrier particles will not be retained by the electroscopic powder particles, while at the same time the electroscopic powder particles are attracted and held by the charged areas of the plate since they acquire a charge of opposite polarity to the charges of both the granular carrier particles and the plate. The degree of contrast in the finished image may be varied by changing the ratio of granular carrier to electroscopic powder. Successful results have been had with 15 parts by weight of granular carrier particles capable of being passed through a 30-mesh screen and being collected on a. 60-mesh screen to 1 part of m'icronized electroscopic powder, although ratios of carrier to powder as great as 100 to 1 have been used with satisfactory results, and it appears that any ratio will operate as long as each carrier particle is covered, or sufficiently covered with electroscopic powder so that the granular carrier particles cannot contact each other, but take on an insulating character.

The granular carrier may consist of materials other than methyl methacrylate, such as sodium chloride, ammonium chloride, aluminum potassium chloride, Rochelle salts, sodium nitrate, aluminum nitrate, potassium chlorate when broken by being passed through a sieve, methyl methacrylate resin, granular zircon. The granular carrier particles may be of any shape within the preferred size range, although it is desirable to have them round or nearly round or uniform so as to facilitate their movement in gravitating over the plate.

While successful results have been had with a mixture of 15 parts of granular carrier material to 1 part of electroscopic powder, this proportion is not critical, and it is only necessary that there be sufficient of the finely ground electroscopic powder to coat a. great number of particles of the coarser granular carrier, which thus becomes an insulator when coated or partially covered with the electroscopic powder, whether the granular carrier particles themselves are conducting or insulating. Ammonium chloride, which is an electrical conductor, can be used as the granular carrier material as successfully as methyl methacrylate, which is an insulating material, because the granular carrier particles in both instances are substantially coated or surrounded by the electroscopic powder and thus take on the characteristic of an insulator.

The granular carrier must have a greater attraction for the electroscopic powder than the essentially discharged areas of the photoconductive layer in order to prevent electroscopic powder particles from adhering to these discharged areas. Thus the electroscopic powder particles are removed with certainty from the discharged areas by the greater attraction of the granular carrier particles and their gravitational movement over the surface, while in the areas where the electrostatic charge remains, the electroscopic powder is retained in uniformly distributed relationship by the greater attraction of the charged areas of the photoconductive insulating layer. The larger and heavier granular carrier particles by their greater weight and momentum separate from and move past the electroscopic powder particles in the charged areas and carry with them the electroscopic powder particles from the discharged areas.

The opposite polarity charges of the electroscopic powder and granular carrier are obtained by proper selection of these materials and bringing them into contact with one another by mixing or otherwise, and the desired polarity of charge of the photoconductive layer as well as its, potential can be selected and controlled so as to obtain the most effective relationship with the electroscopic powder and granular carrier material, and thus bring about the desired results of contrast and clearness in the finished image.

The materials for the electroscopic powder and granular carrier are selected in accordance with their triboelectric properties so that when mixed or brought into mutual contact, one material is charged positively if the other material is below it in a triboelectric series and negatively if the other material is above it in a triboelectric series.

The selection can be made from many materials that have been tested and occupy recognized positions in a triboelectric series so that when mixed, they acquire opposite triboelectric charges, the charge acquired by the electroscopic powder particles having a polarity opposite to that of the charged areas of the photoconductive insulating layer and also opposite to that of the granular carrier particles. By selecting materials in accordance withtheir triboelectric effects the polarities of their charges when mixed are such that the electroscopic powder particles adhere to and encase the granular carrier par-. ticles and also adhere to the electrostatic image om-the plate;. :swhic hqsthns retains ;;the-: electro-i scopic powder in the :charged tareaspthat havev a greater attraction for the electroscopicpowder than th-eigranular carrienparticles have. Either of two selected materials may; be employed" for the eleotroscopic powder or granular material, provided the granularparticles and electrosc'opi-c powderiaregcoarselya and finely ground respecs tively,iasealreadyadescribed. I V .g sflheiprocesszand materialsdescribed above may.

beansed; in COI'IDGOtiOIlJ-With various-types of:

imageebearing plates; as for instance where the plate; does. not" carry a photoconductive layer by coating almetal plate with a solution of proc; ess glue ammonium ,bichromate, and water and permitting the;.coating. to dry Theplate is then placedin a. printing frame with a photographic negative ipositioned ,over the coating layer in contacttherewith. and the plate exposed to intense light-throughthe negative. The plate is developed by'washing with water which removes the unexposed portions of-the coating and leaves an image oi: .insulating.v material consisting of those portionsofithe coating that have been exposed tolighte- The plate is dried and burned in a temperature of about 600 F., whereupon the bare metal surfacemay be cleansed of oxide by means of a; suitable metal cleaner. The image on a plate prepared in this manner canbe charged-and a pewaer image can be formed on the electrically charged image by dusting, in the same ma-nner as already described for the photoconductivejayer plate. 1

7 A plate may be similarly prepared from a coat ing consisting of a shellac emulsion and ammonium bichromate; in which case the plate is developed by washing 'with alcohol, and the imageforming coating remaining after developmentmaybe'hardened"by'baking at a temperature of about 300 F. Such an insulating image-may bezelectrically' charged and dusted to form a powder image "in "the same manner alread de scribed."

A'platewhich will retain an electric charge on the insulating image portion may be pre-'-' pared by coating a-metalplate with a solution of gum arabic and ammonium bichromate in water, Such a coated plate-is dried and can then be eX- fidi entr st.- 51 transparency in a printing; frame,;an d developed by washing in water; A chemical etchant is then applied givinga light etch: togthe metal. At this point in the process the-image portions of the plate arebare metal; Whilethe, areas corresponding to thenon;-image'-;area;s are coated with'gum;arabic The plate--is then, .insea. and dried a'ndnext coated-with'asuitable deep-etch lacquer; which may be, an asphaltunryarnish or similar varnish. After; the lacquer hasr dried, the plate is washed with; hot. wateriwlrich :dissolves'the gum arabic under theilacquericoatingsin the non-image areas I 25. Such an imageebearing plate may be produced and: removesait, together ,w-it:h thqlacquer from: the plate. The image now consists simply thg insulating lacquer coatingon metal-and theplate isr-in; proper condition; f or charging;;and 5 dusting in thepowd'er image transierprocess v Other methods of preparing-suitable image bearing 1 plates have also been suggested. .;-,E or example, a, plate prepared by painting or drawing, an image on metal; with an --insulating;paint would be satisfactory; Similarly, a: conductive plate might beqcoated with an, insulatinglacquer after which certainportions of thelacquermight be removed by scribing, cutting, prengraving to obtain: designs on the plate; In. general-,;, any methodwhich producescja :plate comprisin :a conductive backing with a superimposed non; conductive-image area will besuitableLfonfilfllparing plates fore-use; in: the-.electrical-;chargingand.formation of powder images ashereinbefore described. 1 5 31:1

'It is toxbe understood that the expression .in.-, sulating imagerlayew -where it appears irr-,-'thc specification. and claims refers either toap QtQs conductive insulating layer on which an image is produced by photographic exposure afterjcharging, .or; to an: insulating layer which conforms ins/area tothe desired image and is permanently fixed to aconductive plate, the image in .the lat- 1 ter. instance being produced by charging the insulating layer,

With .this. invention, it hasbeen found that the amount of electricity drained off or the drop in potential in the discharged'areas of thelplateisin proportion to theamount of light projected on-such areas, so that with a uniform charge of 1060 volts over the plate, where maximum. light strikes theplate, the charge in such area will be reduced below say 50 volts,-whereasin areas where less light strikes the plate, such asgrey halftones, the charge willdrop to say 500 volts. The charge on the'plate doesnot drift from one area to another but remains fixed, and consequently since the electroscopic powder is attracted to and retained by the charged areas in density proportional to the charges thereon, the resulting powder image is a true reproduction of the subject being copied, the electrostatic charge and the resulting powder image always-being proportional to the amount of light striking a given area and the degree of opaqueness of the corresponding area inthe subject.-

The mixture of Amberol -F-7l, carbon black, and methyl methacrylate, as above described, is used with a positive charge on the electrophotographic plate, the *electroscopic powderbeing capable of acquiring a negative charge-by' contact with-thephotoconductive material and the granular material, and the granular material beingcapable of acquiring a'positive chargeat the same time by such contact, and other mixtures can be used'with a positively charged plate, as for instance calcium lactate can be mixed with ammonium chloride in the'pr'o'portion-of approxi-' meie y lfim Pr W i ht granular rier tolpart of calcium lactate. v i i:

In cases where the granular carrier is capable of acquir ing'a negative charge, the plate is given a negative charge, and the electroscopic powder iseselectedv so as ,to acquire a positive charge. Sucha result can be had with eitherManjak, an asphaltite found in Eastern Utah and Western Colorado, or Gilsonite, an asphaltite found in the West Indies, pigmented withany suitable ma terial-zasalready described, and..-mixed:with :ame

moniumchloride in the proportion of parts of the-latter to 1 part of the electroscopic powder; or with parts of ammonium chloride to 1 part of a suitably pigmented phenol formaldehyde resin such as known commercially as Amberol BOO-P.

Various pigmenting materials in addition to carbon can be employed and incorporated with. the insulating powder in the manner set forth above, and other materials can be used as the granular carrier such as adipic acid and oxalic acid.

The materials composing the electroscopic powder and granular carrier are selected so that when mixed, the rubbing by frictional contact produces a positive charge on one and a negative charge on the other, and the image-bearing plate is charged to give a polarity opposite to that of the electroscopic powder and the same as that of the granular carrier. While the invention has been described in connection with certain materials and detailed procedures, it is not limited to the exact disclosures herein and this application is intended to cover such departures or substitutions as come within the scope of the following claims.

We claim:

1. A developer composition for developing an electrostatic latent image of substantially homogeneous polarity comprising relatively larger substantially equidimensional carrier particles and grossly smaller colored resin powder particles loosely movable and electrostatically coated thereon, the powder particles being of a size in the order of 0.1 to 20 microns, the carrier particles being of a size such that by their own weight they are movable away from oppositely charged powder particles, the powder particles and carrier particles being of opposite triboelectric characteristics whereby, by mixing, the carrier and powder particles acquire charges of opposite polarity, the powder particles being triboelectrically opposite, with .respect to the carrier particles, to the polarity of the image to be developed and the powder particles by mixing with the carrier particles acquiring a charge of polarity opposite to the image, whereby the powder particles are adapted to be electrostatically removed from the carrier surfaces by and to the charged portions of the electrostatic image and preferentially attracted by and to the carrier surfaces from the background portions of the image.

2. A developer composition for developing an electrostatic latent image of substantially homogeneous polarity comprising relatively larger sub stantially equidimensional carrier particles and grossly smaller colored resin powder particles loosely movable and electrostatically coated thereon, the carrier particles being of a size such that by their own weight they are movable away from oppositely charged powder particles, the powder particles and carrier particles being of opposite triboelectric characteristics whereby by mixing, the carrier and powder particles acquire charges of opposite polarity, the powder particles being triboelectrically opposite, with respect to the carrier particles, to the polarity of the image to be developed and the powder particles by mixing with the carrier particles thus acquiring a charge of polarity opposite to the image, the powder particles being of a size in the order of 6.1 to 20 microns and electrostatically removable from the surface of the carrier particles by contact with a surface having an electrostatic potential greater than about 50 volts of polarity oppo- 10 site to the polarity of the charge on the powder particles.

3. A developer composition for developing an electrostatic-latent image of substantially homogeneous polarity comprising relatively larger substantially equidimensional carrier particles and grossly smaller colored resin powder particles loosely movable and electrostatically coated thereon, the powder particles being of a size in the order of 0.1 to 20 microns, the carrier particles being of a size in the order of 30 to 60-' mesh, whereby, by their own weight they are movable away from oppositely charged powder particles, the powder particles and carrier particles being of opposite triboelectric characteristics whereby, by mixing, the carrier and powder particles acquire charges of opposite polarity, the powder particles being triboelectrically opposite, with respect to the carrier particles, to the polarity of the image to be developed and the powder particles by mixing with the carrier particles acquiring a charge of polarity opposite to the image, the powder particles thereby being electrostatical- 1y removable from the surface of the carrier particles by contact with a surface having an electrostatic potential greater than about volts of polarity opposite to the polarity of the charge of the powder particles.

4. A developer composition for developing a positive polarity electrostatic latent image, comprising relatively larger substantially equidimensional granular carrier particles adapted to roll across a smooth surface, and grossly smaller pigmented thermoplastic resin particles loosely movable and electrostatically coated on the carrier particle surfaces, the carrier particles being of a size in the order of about so to mesh, whereby they are movable by their own weight along and away from oppositely charged surfaces, the powder particles being of a size in the order of 0.1 to 20 microns and triboelectrically negative with respect to the carrier particles, whereby they are adapted to be electrostatically removed from the carrier surfaces to positively charged portions of the electrostatic image and preferentially attracted to the carrier surfaces from background portions of the image.

5. A. developer composition for developing a negative polarity electrostatic latent image, comprising relatively larger substantially equidimensienal granular carrier particles adapted to roll across a smooth surface, and grossly smaller pigmented thermoplastic resin particles loosely movable and electrostatically coated on the carrier particle surfaces, the carrier particles being of a size in the order of about 30 to 60 mesh, whereby they are movable by their own weight along and away from oppositely charged surfaces, the powder particles being of a size in the order of 0.1 to 20 microns and triboelectrically positive with respect to the carrier particles, whereby they are adapted to be electrostatically attracted from the carrier surfaces to negatively charged portions of the electrostatic image and p'eferentially attracted to the carrier surfaces from background portions of the image.

6. A. developer composition for developin a positive polarity electrostatic latent image, comprising relatively larger substantially equidimensional granular carrier particles adapted to roll across a smooth surface, and grossly smaller pigmerited resin particles loosely movable and electrostatically coated on the carrier particle surfaces, the carrier particles being of a size such that they are movable by their own weight along and awayfrom oppositely charged powder parwhereby they are adapted to be electrostatically removed from the carrier surfaces by and to the positively charged portions of the electrostatic image and preferentially removed by the carrier particles from background portions of the image. 7. A developer composition for developing a negative polarity electrostatic latent image, comprising relatively larger substantially equidimensional granular carrier particles adapted to roll across a smooth surface, and grossly smaller pigmented resin particles loosely movable and electrostatically coated on the carrier particle surfaces, the carrier particles being of a size such that they are movable by their own weight along and away from oppositely charged powder particles, the powder particles being of a size in the order of 0.1 to 20 microns and triboelectrically positive, with respect to the carrier particles, whereby they are adapted to be electrostatically removed from the carrier surfaces by and to the negatively charged portions of the electrostatic image and preferentially removed by the carrier particles from background portions of the image. 8. A developer composition for developing an electrostatic latent image of substantially homogeneous polarity comprising relatively larger substantially spherical carrier particles and grossly smaller pigmented resin powder particles loosely movable and electrostatically coated thereon, the powder particles being of a size in the order of 0.1 to 20 microns, the carrier particles being substantially spherical and of a size such that by their own weight they can roll away from oppositely charged powder particles, the powder particles and carrier particles being of opposite triboelectric characteristics whereby, by mixing, the carrier and powder particles acquire charges of opposite polarity whereby upon rolling across a surface bearing an electrostatic latent image the carrier particles are adapted to gain powder particles from the uncharged image areas by electrostatic attraction and to deposit on harged portions of the electrostatic latent image powder particles to form a developed image body thereon.

9. A developer composition for developing an electrostatic latent image of substantially homogeneous polarity comprising substantially spherical carrier particles of a size in the order of 30 to mesh and pigmented resin powder particles loosely movable and electrostatically coated thereon, the powde particles being of a size in the order of 0.1 to 20 microns, the powder particles and the carrier particles being of opposite triboelectric characteristics whereby by mixing, the carrier and powder particles acquire charges of opposite polarity, the powder particles being triboelectrically opposite, with respect to the carrier particles, to the polarity of the image to be developed and the powder particles by mixing with the carrier particles acquiring a charge of polarity opposite to the image, whereby the powder particles are adapted to be electrostatically removed from the carrier surfaces by and to the charged portions of the electrostatic image and preferentially attracted by and to the carrier surfaces from the background portions of the image.

LEWIS E. WALKUP. EDWARD N. WISE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,818,760 Selenyi Aug. 11, 1931 2,192,241 Robijns Mar. 5, 1940 2,221,776 Carlson Nov. 19, 1940 2,297,691 Carlson Oct. 6, 1942 2,345,941 Lehman Apr. 4, 1944 2,357,809 Carlson Sept. 12, 1944 2,463,927 Watts .Mar. 8, 1949 2,484,782 Copley Oct. 11, 1949 2,488,560 Reitlinger Nov. 22, 1949 OTHER REFERENCES Magnetism and Electricity, Brooks & Poyser 1927, Longmans, Green 8: Co. LTD., New York. Page 53. 

1. A DEVELOPER COMPOSITION FOR DEVELOPING AN ELECTROSTATIC LATENT IMAGE OF SUBSTANTIALLY HOMOGENEOUS POLARITY COMPRISING RELATIVELY LARGER SUBSTANTIALLY EQUIDIMENSIONAL CARRIER PARTICLES AND GROSSLY SMALLER COLORED RESIN POWDER PARTICLES LOOSELY MOVABLE AND ELECTROSTATICALLY COATED THEREON, THE POWDER PARTICLES BEING OF A SIZE IN THE ORDER OF 0.1 TO 20 MICRONS, THE CARRIER PARTICLES BEING OF A SIZE SUCH THAT BY THEIR OWN WEIGHT THEY ARE MOVABLE AWAY FROM OPPOSITELY CHARGED POWDER PARTICLES, THE POWDER PARTICLES AND CARRIER PARTICLES BEING OF OPPOSITE TRIBOELECTRIC CHARACTERISTICS WHEREBY, BY MIXING, THE CARRIER AND POWDER PARTICLES ACQUIRE CHARGES OF OPPOSITE POLARITY, THE POWDER PARTICLES BEING TRIBOELECTRICALLY OPPOSITE, WITH RESPECT TO THE CARRIER PARTICLES, TO THE POLARITY OF THE IMAGE TO BE DEVELOPED AND THE POWDER PARTICLES BY MIXING WITH THE CARRIER PARTICLES ACQUIRING A CHARGE OF POLARITY OPPOSITE TO THE IMAGE, WHEREBY THE POWDER PARTICLES ARE ADAPTED TO BE ELECTROSTATICALLY REMOVED FROM THE CARRIER SURFACES BY AND TO THE CHARGED PORTIONS OF THE ELECTROSTATIC IMAGE AND PREFERENTIALLY ATTRACTED BY AND TO THE CARRIER SURFACES FROM THE BACKGROUND PORTIONS OF THE IMAGE. 